脱式计算(能简算的用简便方法计算,写出计算过程.) (1)7.5×2.3-1.9×2.5 (2)(78.6-0.786×25+75%×21.4)×2011÷75 (3)1
(4)9.1+9.2+9.3+…10.7+10.8+10.9. |
(1)7.5×2.3+1.9×2.5
=17.25+4.75
=22;
(2)(78.6-0.786×25十75%×21.4)×2011÷75,
=(78.6-78.6×0.25+75%×21.4)×2011÷75,
=[(1-0.25)×78.6+0.75×21.4]×2011÷75,
=[0.75×78.6+0.75×21.4]×2011÷75,
=(78.6+21.4)×0.75×2011÷75,
=100×0.75×2011÷75,
=75×2011÷75,
=75÷75×2011,
=2011;
(3)1
+21 2
+31 6
+41 12
+51 20
,1 30
=(1+2+3+4+5)+(
+1 2
+1 6
+1 12
+1 20
),1 30
=15+[(1-
)+(1 2
-1 2
)+(1 3
-1 3
)+(1 4
-1 4
)],1 5
=15+[1-
+1 2
-1 2
+1 3
-1 3
+1 4
-1 4
],1 5
=15+
,4 5
=15.8;
(4)9.1+9.2+9.3+…10.7+10.8+10.9,
=(9.1+10.9)×9+10.0,
=180+10.0,
=190.